Chitin is a naturally occurring linear polymer of N-acetylglucosamine, which is a derivative of glucose. Additionally, chitin, which can be found as the structural support in the outer skeleton of arthropods and in the cell wall of fungi, is the second-most abundant naturally occurring polymer in the world (Bartlett et al., Science, 310:1775-1777, 2005). Chitin's high bioactivity, biocompatibility, strength, and low toxicity (Jayakumar et al., Garb. Polym., 82:227-232, 2010), make it suitable for a variety of both low value applications, such as water treatment, agriculture and food additives, and high value applications, such as cosmetics, medical bandages, and drug delivery.
However, obtaining pure chitin has remained elusive due to its insolubility in water, most organic solvents, dilute acidic solutions, and dilute basic solutions. Various modifications have been utilized to make chitin more water soluble, including partial N-deacetylation, through the removal of the acetyl functional group, to form chitosan (Rinaudo, Polym. Sci., 31:603-632, 2006). The material properties can change as a function of the degree of acetylation (DA), which is a measure of the ratio of acetyl groups to amine groups (FIG. 1).
The current industrial pulping process for chitin from crustacean shell involves two basic steps: (i) deproteinization, removal of proteins and lipids, by alkaline treatment (NaOH, 1 M, 1-72 h, 65-100° C.) and (ii) demineralization, separation of calcium carbonate/calcium phosphate, by acidic treatment (HCl, 0.275-2 M, 1-48 h, RT-100° C.) (Percot et al., Biomacromolecules, 4:12-18, 2003) and Mahmoud et al., Biochem. Biotechnol., 3:1-9, 2007). Such extreme conditions can result in hydrolysis/degradation of the chitin, allowing for little control over the final product's physical characteristics, such as crystallinity, purity, and polymer chain arrangement.
Strong acid treatment can affect the polymer backbone of the chitin molecule, as it can cleave the β-1,4-glycosidic bonds in chitin, which can result in a lower molecular weight and, upon dissolution, a lower viscosity. The strong acids used for the demineralization of chitin can also affect the degree of acetylation (Cauchie, In Advances in Chitin Science, A. Domard, G. A. F. Roberts, K. M. Varum (Eds.), 1997, pp. 32-38), which can severely limit the potential applications of the chitin. Various chemical modifications of the above-mentioned industrial protocol have been applied to disrupt the inter- and intramolecular hydrogen bonds of the chitin without cleavage of its glycosidic linkages (Sashiwa and Shigemasa. Carbohydr. Polym., 39:127, 1999). This process of chitin purification is not only energy-intensive, but damaging to the environment because of the disposal of high volumes of mineral acids and bases. This high volume of corrosive waste streams results in increased costs and regulatory scrutiny.
Thus, not only do current methods of isolating chitin by chemical pulping lead to partial deacetylation and decrease of molecular weight, but they are costly and energy intensive. As a result, there is a need for a cheaper and more effective process for pulping chitinous biomass for pure, high quality chitin material with a high percentage of acetylation and high molecular weight. The methods and compositions disclosed herein address these and other needs.